An organic light emitting diode (OLED) display device has potential in development in the future. This is because a liquid crystal display (LCD) device requires a backlight module as a light source so that images can be displayed, in contrast, the organic light emitting diode display device can emit lights by itself and therefore does not require the backlight module. However, if thin film transistors (TFTs) in the organic light emitting diode display device are manufactured by a semiconductor material having a low carrier mobility, such as hydrogenated amorphous silicon (a-Si:H), current provided by the thin film transistors will be insufficient. This situation will contribute to require higher cost to design a compensating circuit.
A structure of a vertical thin film transistor has a high carrier mobility and thus the above-mentioned situation of the current insufficiency can be avoided. In the past researches and studies about the vertical thin film transistor, one famous paper was proposed by Fujimoto et al. in 2007 (Fujimoto Kiyoshi, Takaaki Hiroi, Kazuhiro Kudo, and Masakazu Nakamura, “High-Performance, Vertical-Type Organic Transistors with Built-in Nanotriode Arrays”, Advanced Materials, 19, 525, 2007). Please refer to FIGS. 1A-1D. FIGS. 1A-1D illustrate a manufacturing process flow chart of the vertical thin film transistors 100 proposed in the above-mentioned paper. In FIG. 1A, numerous polystyrene particles 120 having charge with the same polarity are sprayed on a glass substrate 102. The polystyrene particles 120 are used as shutters for separating transistor elements which will be manufactured on the glass substrate 102. In theory, there is a distance between the adjacent polystyrene particles 120 because of repulsive force of the charge with the same polarity. In FIG. 1B, the polystyrene particles 120 are served as evaporation masks, and a first electrode layer 104 served as a drain layer (or a source layer), an insulating layer 106, and a gate electrode layer 108 are deposited sequentially between the adjacent polystyrene particles 120. In FIG. 1C, the polystyrene particles 120 are removed by using an adhesive tape 130. In FIG. 1D, a semiconductor layer 110 and a second electrode layer 112 served as a source layer (or a drain layer) are deposited sequentially to complete manufacturing of the vertical thin film transistors 100.
However, there are some problems in the above-mentioned manufacturing method. First, numerous amounts of the polystyrene particles 120 sprayed on different areas of the glass substrate 102 are difficult to control. Thus, the polystyrene particles 120 are usually distributed non-uniformly thereon. Accordingly, the transistor elements manufactured on the glass substrate 102 are also distributed non-uniformly. Furthermore, the distance between the adjacent polystyrene particles 120 is proportional to the quantities of charge of the adjacent polystyrene particles 120. That is, when the quantities of charge of the adjacent polystyrene particles 120 are great, the distance between the adjacent polystyrene particles 120 is long. Because the polystyrene particles 120 have different quantities of charge, the distances between the adjacent polystyrene particles 120 are also different. As a result, the transistor elements manufactured on the glass substrate 120 have different sizes, and characteristics of the transistor elements are also different, resulting in difficulty in controlling the transistor elements.
Please refer to FIG. 2. FIG. 2 illustrates a structure of the vertical thin film transistor 100 and a driving circuit thereof in the above-mentioned paper. The vertical thin film transistor 100 manufactured by the manufacturing method shown in FIGS. 1A-1D is capable of providing a sufficient current to drive organic light emitting diodes when VDS and VG are both low. Accordingly, the vertical thin film transistor 100 is suitable to be utilized in the organic light emitting diode display device without an extra compensating circuit. A depletion region, which is formed in a contact interface between the gate electrode layer 108 and the semiconductor layer 110, is served as an insulating layer in the vertical thin film transistor 100. However, the depletion region is very small, and therefore VG applied to the vertical thin film transistor 100 can not be too high. Other problems such as a high off-state current and a low ON/OFF current ratio of only approximately 103, lead to a bad performance on switching effects of the vertical thin film transistor 100.
Therefore, there is a need for a solution to the above-mentioned problems of the vertical thin film transistor.